The present invention relates to digital watermarking systems and methods, and is particularly illustrated with reference to digitally watermarking spot colors.
Digital watermarking technology, a form of steganography, encompasses a great variety of techniques by which plural bits of digital data are hidden in some other object, preferably without leaving human-apparent evidence of alteration.
Digital watermarking may be used to modify media content to embed a machine-readable code into the media content. The media may be modified such that the embedded code is imperceptible or nearly imperceptible to the user, yet may be detected through an automated detection process.
There are many processes by which media can be processed to encode a digital watermark. Some techniques employ very subtle printing, e.g., of fine lines or dots, which has the effect slightly tinting the media (e.g., a white media can be given a lightish-green cast). To the human observer the tinting appears uniform. Computer analyses of scan data from the media, however, reveals slight localized changes, permitting a multi-bit watermark payload to be discerned. Such printing can be by ink jet, dry offset, wet offset, xerography, etc. Other techniques involve varying the luminance of pixel colors.
Digital watermarking systems typically have two primary components: an embedding component that embeds the watermark in the media content, and a reading component that detects and reads the embedded watermark. The embedding component embeds a watermark pattern by altering data samples of the media content. The reading component analyzes content to detect whether a watermark pattern is present. In applications where the watermark encodes information, the reading component extracts this information from the detected watermark. Previously mentioned U.S. patent application Ser. No. 09/503,881, filed Feb. 14, 2000, discloses various encoding and decoding techniques. U.S. Pat. Nos. 5,862,260 and 6,122,403 disclose still others. Each of these patents is herein incorporated by reference. The reader is presumed familiar with the many other digital watermarking techniques.
Sometimes an image is printed using xe2x80x9cspot colorxe2x80x9d inks instead of process inks (e.g., cyan, magenta, yellow and black, hereafter xe2x80x9cCMYKxe2x80x9d). Spot colors can be produced in a vibrant range of colors, and can have characteristics that are not generally available in process inks, such as day-glow or metallic ink among others. There are thousands of spot colors. Yet, the color gamut (or range) of CMYK is limited. This means that most spot colors, when converted to a CMYK approximation, will not match the original spot color. This presents a problem when attempting to embed a digital watermark within the spot color. One digital watermarking approach produces an approximation of the spot color image using only CMY or CMYK inks. A digital watermark signal is embedded through modulating (or changing) various CMYK pixels. Since the original spot color may lie outside the CMYK gamut, this first spot color watermarking approach can yield poor results.
The present invention provides various methods to effectively embed a digital watermark signal within a spot color. While our inventive processes can be applied to a variety of spot color printing techniques, our processes are particularly helpful for embedding spot color digital files (e.g., electronic representations of the spot color or digital spot color profiles, etc.). A spot color digital file generally includes a set of intensity values indicating an amount or percentage of ink to be applied to a pixel or to a print area. A printing plate (or other printing map) is produced in accordance with the digital file. Spot color ink is applied to the printing plate to facilitate printing.
In a first aspect of the present invention, a digital watermark signal is converted from a set of luminance tweaks (e.g., signal modifications or a message, etc.) to a set of intensity tweaks. This watermark intensity set is provided to modulate (or alter) the spot color digital file.
According to a second aspect, noise introduced from an image capture device (e.g., a digital camera, scanner, etc.) is compensated for through intensity modulation of a digital watermark signal. In one implementation of this aspect, color saturation is calculated as a modulation factor to offset the noise.
According to a third aspect of the present invention, a spot color detection method authenticates a spot color. A light emission from a spot color is separated into its constituent spectrum components. A signature is determined based on the constituent spectrum components. This signature is used to identify counterfeits. For example, a CMYK approximation of the spot color will yield a different signature in comparison to the original spot color.
The foregoing and other features and advantages of the present invention will be even more apparent from the following detailed description, which proceeds with reference to the accompanying drawings.